1.4- Dioxocine

Besides l,4-dihydro-l,4-diazocines (see Section 1.4.) there are other 1,4-diheterocines that are isoelectronic with the cyclooctatetraene dianion. These systems have primarily found interest as potential 107t-aromatic systems. No extensive delocalization is expected theoretically and calculations predict nonaromatic systems for both 1,4-dioxocin (1) and 1,4-dithiocin (see Section l.ll.).3-4-7... 

In contrast to the 1,4-dithiocin system, 1,4-dioxocin (1) is well-known and has been characterized as an olefinic compound by its spectra as well as its chemical behavior.5-6 The reason why 1,4-dioxocin in contrast to 1.4-dihydro-1.4-diazocine (see Section 1.4.) and 4//-l,4-oxazocinc (sec Section 1.12.), does not qualify as a 107r-aromatic species, is the less pronounced tendency of oxygen atoms for 7t-electron delocalization. An X-ray analysis of the 6-substituted 1,4-dioxocin 2 confirms the presumed nonplanar conformation of the 1,4-dioxocin structural element.9 The eight-membered ring exhibits a twisted boat-chair confirmation. 

Other substituted systems, however, might be planar due to conjugation effects with acceptor substituents, as has been found in an X-ray structural analysis of 1,4-dioxocin-6-carboxylic acid chloride the eight-membered ring is practically planar with a coplanar arrangement of the substituent.9... 

Depending upon the substitution pattern, a thermal valence isomerization of 1,4-dioxocins 4 to the tricyclic jyn-benzene dioxides (xyn-3,8-dioxatricyclo[5.1,0.02-4]oct-5-enes) 3 can be detected. On the other hand, the valence isomerization of sin-benzene dioxides (anti-benzene dioxides do not undergo such rearrangements) provides a general approach to 1,4-dioxocins 4. 

The 6-substituted 1,4-dioxocins can be used to prepare other 6-substituted derivatives by simple functional group transformations.4,8,9 Especially interesting is the synthesis of the 4/7-4-oxo-2,3-dihydropyran-2-yl-substituted derivative 16 from l,4-dioxocin-6-carbaldehyde (15) by a cyclocondcnsation with Danishefsky s diene.9 Dehydrogenation of 16 yields 2 which can be isomerized to the corresponding isomeric. sr/i-benzene dioxide 3 (X = 4/f-4-oxopyran-2-yl), which is identical with and proved the structure of the naturally occurring antibiotic LL-Z 1220.10... 

Dioxocines 218 are in turn paratropic and exist in equilibrium with their 2o — 2ti valence isomers sjn-benzene dioxides 217 (Scheme 80).259 Their chemistry and behavior in magnetic fields have been evaluated.260 261... 

Dioxatricyclo[3.3.0.02-4]octenes 1 underwent [2 + 2] cycloreversion on flash-vacuum thermolysis at 550 CC and 0.003 Torr to give 1,4-dioxocins 2 in addition to other products.191... 

SINDOl calculations, which successfully reproduced both the geometry and the aromaticity of the cyclooctatetraene dianion, predict a high degree of bond localization and nonplanarity for 1,4-dioxocin and its derivatives (84JOC4475). 

The parent 1,4-dioxocin (15) is colorless, with its major absorption band well into the ultraviolet [238 nm (3.86)] and a weak shoulder at 285 nm (2.50), a spectrum similar to that of 1,3,6-cyclooctatriene (72AG(E)935). 

Dioxocine (32 X = 0) behaves chemically as an alkene rather than as an aromatic compound. Thus, it is readily hydrogenated to 1,4-dioxocane and polymerizes readily upon standing (72AG(E)935>. 

Table I lists NMR information that proved useful in understanding the -electronic requirements of the basic 1,4-diheterocin system. The 1,4-dioxocin molecule (2a) may thus be classed as nonaromatic,7 based on the close similarity of its proton shifts with those displayed by the family s monohetero polyenic models (4a and b), whereas the fact that...

E. Vogel, H.-J. Altenbach, and D. Cremer, Angew. Chem., Int. Ed. Engl. 11,935 (1972). This paper also describes the conversion of bromide (5a) to parent 1,4-dioxocin by tri-n-butyltin hydride. 

In the crystalline state syw-benzene diepoxide exists only as benzene diepoxide 55 at room temperature. However, on heating it undergoes symmetry allowed 2S + a2s + a2s cycloreversion to 1,4-dioxocin (166).58... 

The ring systems 67 are formally 10-electron systems and aromatic if planar. In practice, the geometry is dependent on the nature of the heteroatom. As might be expected from a comparison with furan, 1,4-dioxocin 67 (X = 0) behaves chemically as an alkene rather than as an aromatic compound. It is readily hydrogenated to 1,4-dioxocane, tautomerizes to benzene dioxide 35, and polymerizes readily upon standing <1972AGE935>. 

Dioxocines 4 are paratropic and exist in equilibrium with their Zcr—> 2ti valence isomers, ry -benzene dioxides 5 (Equation 1) <1984CHEC(5)653, 2000CC2151, 2004CR2777> (see Section 14.06.6.2). 

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